Optical fibers are waveguides that transmit light, with minimal scattering and attenuation, between two locations. Optical fibers, also sometimes called fiber optics, are well known and used for illumination, communications, information transfer, and sensors, for example. Optical fibers are typically flexible and very thin, i.e., on the order of less than the thickness of a human hair. They have a transparent core and one or more transparent cladding layers. The core and cladding layers are made of vitreous material, such as high quality glass (made from, e.g., silica, fluoride, phosphates, etc.) or even certain plastics. Moreover, the core material has a refractive index which is greater than the refractive index of the material in the surrounding cladding layer or layers. These conditions enable total internal reflection of light signals passing through the fiber, resulting in an efficient waveguide.
Optical fibers are generally manufactured by drawing the fiber from a heated preform using a fiber drawing tower. Such towers are typically vertically oriented and have a guide to hold and guide a preform, end first, into the top of the tower, as well as a high temperature furnace to heat the preform in a controlled manner, and apparatus to apply controlled tension to the leading end of the preform, whereby a fiber of molten material forms. The fiber is typically cooled and solidified as it is drawn from the preform to provide a fine continuous optical fiber.
The preforms are generally cylindrical or tubular in overall shape, with circular cross-sectional profiles, but may have other cross-section profiles (e.g., oval, elliptical, angular, etc.). Like the fibers which are drawn from them, preforms have an axial core of transparent vitreous material which is selected and formulated to provide the particular light transmitting properties needed, such as refractive index, attenuation, etc., according to its intended end uses. The preform core is completely surrounded and enclosed by at least one cladding layer which is also made of transparent vitreous material, but which has a lower refractive index than that of the core.
There are several techniques practiced for making optical preforms, most of which involve one or more types of chemical vapor deposition (CVD), including inside vapor deposition, outside vapor deposition and vapor axial deposition. All such techniques generally involve depositing one or more layers of soot material onto a substrate, followed by high temperature heating to vitrify the deposited soot materials into solid glass. The substrate may be a rod of material that can withstand the subsequent high temperature heating, or it may be a previously deposited layer of soot material, or a previously formed and vitrified rod or layer of glass, or even some combination of these. In some cases, the heating step which vitrifies the layered soot material may be performed immediately prior to, or concurrently, with heating of the vitrified preform and drawing of the fiber, so that formation of the preform and formation of the fiber are sequential and continuous.
CVD techniques may be combined with more recently developed processes, known as rod-in-tube (RIT) and rod-in-cylinder (RIC), for manufacturing optical preforms. RIT and RIC methods both start with a core glass rod and a glass jacket (either a tube or cylinder). The core glass rod has a core and a primary cladding layer, both of which are transparent, and the refractive index of the core is greater than that of the primary cladding layer. The glass jacket provides a second layer of glass cladding material, also sometimes referred to as “overcladding.” The glass jacket may be a large outer diameter cylinder, or the cylinder may be drawn into a smaller outer diameter tube, both of which have an axial opening sized to receive the core glass rod. The core glass rod and glass jacket are produced separately and then assembled by insertion of the core rod into the tube or cylinder, followed by heating and solidification in a vertically-oriented jacketing apparatus similar to the fiber drawing tower, to form a solid vitreous preform. The preform is then be fed to a fiber drawing tower, where it is heated and the optical fiber drawn from the leading edge of the heated preform.
It is clear that careful control of the amounts of soot materials deposited during CVD methods, as well as control of the dimensions (inner diameter, outer diameter, length, etc.) of the core rod and the tube or cylinder jackets before and during vitrification, and the diameter and thickness of the final optical fiber and its layers, are all of critical importance to ultimately producing optical fibers having the desired properties and quality. Therefore, there are various known methods and apparatus for measuring the various properties of the components of the preform, the preform itself, and the optical fiber during different stages of the overall manufacture process.
For example, the diameter of optical preforms, whether made by CVD, RIT, RIC, or other methods, is typically measured after vitrification, but before beginning the drawing of optical fibers therefrom. One such method involves directing a laser or other radiation beam at a preform, where the laser or beam is at a right angle to the longitudinal axis of the preform and is reciprocated back and forth from one side of the preform to the other, linearly in a plane also oriented at a right angle to the longitudinal axis of the preform. The image produced by such methods is recorded and analyzed to produce and report at least the outer diameter of the preform, and sometimes also the diameters of the core and cladding layers, as well as other information. Often, the image is captured by an image sensor, such as a charge-coupled device (“CCD”) image sensor, and recorded as a digital image for manipulation and interpretation by a processor. Laser scanning methods where the image sensor is located on the opposite side of the preform from the laser source and receives the shadow image produced as the laser shines through the preform are known as the shadow technique.
There are limitations to the shadow measurement of diameter measurement, some of which are imposed by features of the apparatus. For example, typically the furnace apparatus in which heating and vitrification of the preform occur has an opening which is sized and shaped, often as a long narrow rectangle or slit, to accommodate the reciprocating scanning laser passing through the opening to the preform. Preforms produced by the RIT method can be manufactured having outer diameters (OD) up to about 110 millimeters (mm), and that diameter could be accurately measured using telecentric laser scanning gauges and image sensors appropriately installed and operated proximate to the furnace apparatus. Of course, due to the nature of laser scanning technology, the rectangular window or slit must be a bit wider than the widest preform to be measured and, for a time this was possible. However, customer demand for larger diameter preforms led to the development of the RIC method for manufacturing preforms having diameters between about 135 and 150 (mm), and even larger, and limitations of the geometry and function of the apparatus and the laser scanner technology required development of a new way to measure larger diameter preforms and equipment for practicing such a method.
The present invention provides a novel apparatus and method for measuring the diameter of optical preforms, or other optically transparent cylindrical articles, during their manufacture. The novel apparatus includes use of a digital camera having a lens and a image receiver and recorder, in place of the previously employed laser scanning apparatus, while the method employs an algorithm developed to analyze the image received by the image sensor in a manner which eliminates noise and redundancy in the image to determine the preform diameter.